bpf_xdp_pointer
void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len);
void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset + offset, len);
ptr = bpf_xdp_pointer(xdp, offset, len);
ptr = bpf_xdp_pointer(xdp, offset, len);